Air consumption in a railway car is primarily the result of the above-mentioned pneumatically-operated equipment. However, its consumption varies considerably depending on the operating condition of the railway car (express or the local which stops at every station), the time (rush hour or non-rush hour) and the route condition (slope, curve and distance between the stations), etc. Normally, the rate of operation of the air compressor is approximately 30%; however, it changes to approximately 20-40% depending on the conditions described above. Despite the high or low air consumption, the operation of the air compressor in the prior art is always operated at the rated level (full operation), and its sudden starting causes a reaction in the car body by the inertial force of the motor and the compressor, so that, in some cases, it can cause vibration of the car body and potential discomfort to the passengers. At the same time, the vibration of the compressor itself is significant, so that the piston rings or ball bearings will generally wear out rapidly. In addition, the frequency of starting and stopping of the compressor is high and its start-up current is high, so that electrical power consumption increases; furthermore, the principal contactor rapidly experiences wear, and the compressor starting device consisting of the principal contactor, the series resistor and breaker, becomes quite large.
The prior art includes Japanese Utility Model No. 60-16971, and the following is a detailed explanation of the pressure adjustment method of this particular prior art system, with reference to the accompanying FIGS. 3 and 4.
FIG. 3 is a graph which indicates the air pressure in the air reservoir and the starting and the stopping of the railway car air supply system air compressor, and FIG. 4 is a schematic diagram of the air supply system including the pressure adjustment device.
As can be seen in FIG. 4, the system includes an air compressor 1, and air reservoir 2, a check valve 3, in which the forward direction is the direction of the air reservoir 2. An air adjustment device 4 is provided which detects the air pressure P in the air reservoir 2. In this prior art air supply system, the driving device 5 starts and stops the air compressor 1 depending on the electrical signal coming from the pressure adjustment device 4. The pressure adjustment device 4 is electrically designed so that it can have the same function as the pressure switch indicated in the above-mentioned Japanese Utility Model Patent No. 60-16971. The pressure adjustment device 4 consists of the sensor 11 which converts the air pressure P in the air reservoir 2 to the electrical signal E11, of the deformation gauge. The comparison part 17 has a hysteresis characteristic and makes the output E17 (L) when the air pressure P rises to the upper limit set point P4 and makes the output E17 (H) when the air pressure P decreases to the lower limit set point P1. The output part 18 is operated by the output E17 of this comparison part 17, and is equipped with a transistor or relay, etc. Furthermore, both above-mentioned set points P1 and P4 can be determined freely by the variable resistor VR1, VR4, of the comparison part 17. In addition, the drive device 5 includes a principal contactor (relay) or a series resistor, etc. In addition, the dust filter, the muffler, the aftercooler, and the moisture remover, etc. (designated "auxiliaries" below) are located around the air compressor 1, but they are not illustrated in the figure. In FIG. 3, time t0-t11 indicates the first charging time. When the power is supplied from the power source (not shown in the figure) at the time t0, since the air pressure P in the air reservoir 2 is atmospheric, the pressure adjustment device 4 detects this condition and the air compressor 1 is operated at its rated capacity, and the air pressure P rises. When the air pressure P reaches the upper limit set point P4 at time t11, the pressure adjustment device 4 detects this, and the air compressor 1 stops. After that, the air pressure P decreases due to the air consumption by the pneumatic equipment, and when it becomes less than the lower limit set point P1 at time t12, the pressure adjustment device 4 detects this condition, and the air compressor 1 is once again operated at its rated capacity. Thus, by starting and stopping the air compressor 1, the air pressure P in the original air reservoir 2 is adjusted between the lower limit set point P1 and the upper limit set point P4.